It is known that a single crystal of potassium titanium phosphate, which has the same crystal structure as that of potassium titanium arsenate, is able to convert an infrared YAG laser ray into a green ray and that the conversion efficiency is high. Recently, it has been found that the conversion efficiency of a single crystal of potassium titanium arsenate is higher than that of a single crystal of potassium titanium phosphate and therefore the former single crystal is widely noticed by those skilled in the art.
A single crystal of potassium titanium arsenate (KTiOASO.sub.4) is manufactured by hydrothermal synthesis method under severe conditions of a temperature of from 500.degree. to 700.degree. C. and a pressure of 3000 atm or so. This is because if KTiOAsO.sub.4 is directly heated up to a high temperature, it would decompose and the arsenic acid radical would be evaporated out therefrom and, as a result, the chemical composition of the crystal to be obtained would be varied. Therefore, a single crystal of KTiOAsO.sub.4 is inevitably high-priced so that it is not utilized industrially at all despite of the extremely excellent characteristic thereof.